Stent with eased corner feature

ABSTRACT

An implantable stent includes a plurality of rings. At least a distal end ring has an eased corner feature formed in the polymer substrate at a radially outward, distal-facing corner of the ring while relatively sharp corners of the polymer substrate are maintained in radially inward corners of the ring.

FIELD OF THE INVENTION

This invention relates generally to implantable medical devices and,more particularly, to polymeric stents.

BACKGROUND OF THE INVENTION

Stents are frequently used in the medical field to open vessels affectedby conditions such as stenosis, thrombosis, restenosis, vulnerableplaque, and formation of intimal flaps or torn arterial linings causedby percutaneous transluminal coronary angioplasty (PCTA). Stents areused not only as a mechanical intervention, but also as vehicles forproviding biological therapy. As a mechanical intervention, stents actas scaffoldings, functioning to physically hold open and, if desired, toexpand a vessel wall. Stents may be capable of being compressed indiameter, so that they can be moved through small vessels with the useof a catheter or balloon-catheter, and then expanded to a largerdiameter once they are at the target location. Examples of such stentsinclude those described in U.S. Pat. No. 4,733,665 to Palmaz, U.S. Pat.No. 4,800,882 to Gianturco, U.S. Pat. No. 4,886,062 to Wiktor, U.S. Pat.No. 5,514,154 to Lau et al., and U.S. Pat. No. 5,569,295 to Lam.

A stent must have sufficient radial strength to withstand structuralloads, such as radial compressive forces, imposed on the stent as itsupports the walls of a vessel or other anatomical lumen. In addition,the stent must possess sufficient flexibility to allow for crimping,deployment, and cyclic loading from surrounding tissue. Also, asufficiently low profile, that includes diameter and size of struts, isimportant. As the profile of a stent decreases, the easier is itsdelivery through an anatomical lumen, and the smaller the disruption inthe flow of blood or other bodily fluid.

Stents made of bioresorbable polymers have been developed to allow forimproved healing of the anatomical lumen. Examples of bioresorbablepolymer stents include those described in U.S. Pat. No. 8,002,817 toLimon, U.S. Pat. No. 8,303,644 to Lord, and U.S. Pat. No. 8,388,673 toYang. FIG. 1 shows an end segment of an exemplary bioabsorbable polymerstent 10 designed to be delivered through anatomical lumen using acatheter and subsequently expanded. Stent 10 has a cylindrical shapehaving central axis 12 and includes a pattern of interconnectingstructural elements or struts 14. Axis 12 extends through the center ofthe cylindrical shape formed by struts 14. The stresses involved duringcompression and deployment are generally distributed throughout variousstruts 14 but are focused at the bending elements or strut junctions.

There are different types of struts 14. Struts 14 include a series ofring struts 16 that are connected to each other by bending elements 18.Ring struts 16 and bending elements 18 form sinusoidal rings 20configured to be reduced and expanded in diameter. Rings 20 are arrangedlongitudinally and centered on axis 12. Struts 14 also include linkstruts 22 that connect rings 20 to each other. Rings 20 and link struts22 collectively form a tubular scaffold of stent 10. Ring 20 d islocated the distal end of stent 10.

Bending elements 18 form a more acute angle when stent 10 is crimped toallow radial compression of stent 10 in preparation for delivery throughan anatomical lumen. Bending elements 18 subsequently bend to form alarger angle when stent 10 is deployed to allow for radial expansion ofstent 10 within the anatomical lumen. After deployment, stent 10 issubjected to static and cyclic compressive loads from surroundingtissue. Rings 20 are configured to maintain the expanded state of stent10 after deployment.

Polymer stents are typically more flexible than metallic stents. Whilegreater flexibility facilitates deliverability through tortuousanatomical lumen, flexibility of the polymer substrate material alsorequires individual struts of polymer stents to be thicker than strutsof comparable metallic stents in order to meet requisite mechanicalstrength requirements. Thicker struts can result in a polymer stenthaving a larger stent scaffold profile, or outer diameter, duringdelivery through an anatomical lumen.

In some cases, calcification can be present on the interior surface ofan anatomical lumen. Due to a larger stent scaffold profile and higherconformability of the scaffold against the walls of the anatomicallumen, sharp edges of polymer stent struts may catch on spicules ofcalcium present in the anatomical lumen. However, making stent strutsthinner in an effort to reduce the chance of catching calcium onanatomical lumen walls, can impact the mechanical strength of the stent.

Accordingly, there is a continuing need for stent strut configurationsand manufacturing methods that facilitate delivery of polymer stents.

SUMMARY OF THE INVENTION

Briefly and in general terms, the present invention is directed to animplantable stent and a method for making an implantable stent.

In aspects of the present invention, an implantable stent comprises afirst ring and a second ring, each of the first ring and the second ringmade of a polymer substrate material forming a strut abluminal surface,a strut luminal surface, a distal-facing strut side surface, and aproximal-facing strut side surface. The distal-facing strut sidesurfaces meet strut luminal surfaces to form sharp radially inwardcorners on the first ring and the second ring, an eased corner featureis formed in the polymer substrate material of the first ring, and theeased corner feature is located at a radially outward, distal-facingcorner of the first ring and is configured to ease movement of the firstring through an anatomical lumen.

In aspects of the present invention, a method comprises providing atubular construct made of a polymer substrate material, and removingpolymer substrate material from the tubular construct to form a firstring and a second ring. The removing of the polymer substrate materialincludes forming sharp radially inward corners on the first ring and thesecond ring, and forming an eased corner feature at a radially outward,distal-facing corner of the first ring.

Any one or a combination of two or more of the following can be appendedto the above aspects of the invention to form additional aspects of theinvention.

The eased corner feature has an eased width and an eased depth, theeased width is at least 5% of strut width, and the eased depth is atleast 5% of strut height.

The first ring is a distal end ring.

An eased corner feature is formed in the polymer substrate material ofthe second ring and is configured to ease movement of the second ringthrough an anatomical lumen.

The eased corner feature of the second ring is located on aradially-outward, distal-facing corner of the second ring.

The eased corner feature of the second ring is located on aradially-outward, proximal-facing corner of the second ring.

The first ring is a distal end ring, and second ring is a proximal endring.

A third ring is disposed between the first ring and the second ring, thethird ring configured to radially expand and made of a polymer substratematerial forming a strut abluminal surface, a strut luminal surface, adistal-facing strut side surface, and a proximal-facing strut sidesurface, wherein the distal-facing strut side surface andproximal-facing strut side surface meet the abluminal surface to formsharp radially outward corners on the third ring.

A plurality of additional rings each of which made of the polymersubstrate material, wherein an eased corner feature is formed in thepolymer substrate material at radially outward, distal-facing corner ofeach of the additional rings, and the eased corner feature is configuredto ease movement of each of the rings through an anatomical lumen.

The first ring includes ring struts joined together by a proximalbending element and a distal bending element, wherein the ring struts,the proximal bending element, and the distal bending element form asinusoidal shape, wherein a width ratio is a ratio of eased width of theeased corner feature to strut width, and the width ratio of the firstring is the same at the proximal bending element and at the distalbending element, and an eased depth of the eased corner feature of thefirst ring is the same at the proximal bending element and at the distalbending element.

The eased width at the proximal bending element and at the distalbending element is less than 100% of strut width.

The eased width at the proximal bending element and at the distalbending element is 100% of strut width.

Strut height is greater at the proximal bending element than at thedistal bending element, and the strut width is the same at the proximalbending element and at the distal bending element.

Strut height is greater at the proximal bending element than at thedistal bending element, and the strut width is less at the proximalbending element than at the distal bending element to allow for uniformexpansion of the first ring.

The eased corner feature of the first ring includes a flat surfaceoriented at a bevel angle less than 90 degrees relative to thedistal-facing strut side surface.

The eased corner feature of the first ring includes a convex surface.

Sharpness of a radially outward, proximal-facing corner of the firstring is maintained during forming of the eased corner feature at theradially outward, distal-facing corner of the first ring.

Forming of the sharp radially inward corners and forming of the easedcorner feature are performed using the same cutting tool.

Forming of the sharp radially inward corners and the forming of theeased corner feature are performed using different cutting tools.

The cutting tool used to form the eased corner feature is selected fromthe group consisting of a laser, a water jet, a knife, a buffing wheel,a jet of abrasive material, and a combination of two or more thereof.

Forming of the eased corner feature includes removing polymer substratematerial from the tubular construct using the cutting tool.

Forming of the eased corner feature includes removing polymer substratematerial from the tubular construct using a thinning tool followed byusing the cutting tool.

Forming of the sharp radially inward corners is performed using a firsttool, and is followed by the forming of the eased corner feature using asecond tool.

The features and advantages of the invention will be more readilyunderstood from the following detailed description which should be readin conjunction with the accompanying drawings.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in the presentspecification are herein incorporated by reference to the same extent asif each individual publication or patent application was specificallyand individually indicated to be incorporated by reference. To theextent there are any inconsistent usages of words and/or phrases betweenan incorporated publication or patent and the present specification,these words and/or phrases will have a meaning that is consistent withthe manner in which they are used in the present specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a polymer stent.

FIG. 2 is a cross-section view of a polymer stent, showing the stentmounted on a catheter.

FIG. 3 is a cross-section view of a polymer stent mounted on a catheter,showing an eased corner feature on a distal end ring of the stent.

FIG. 4A is a perspective view showing the distal end ring of FIG. 3before the distal end ring is crimped onto a balloon catheter.

FIG. 4B is a cross-section view along line 4B-4B in FIG. 4A.

FIG. 5 is a cross-section view of a stent strut, showing a bevel-typeeased corner feature.

FIG. 6 is an elevation view of an apparatus for fabricating a stent,showing a cutting tool for forming an eased corner feature.

FIG. 7 is a cross-section view of a stent strut, showing a laser beamperpendicular to the stent strut abluminal surface.

FIG. 8 is a cross-section view of a stent strut, showing a laser beam atan oblique angle to the stent strut abluminal surface.

FIG. 9 is a cross-section view of a polymer stent mounted on a catheter,showing an eased corner feature on a distal end ring of the stent.

FIG. 10A is a perspective view showing the distal end ring of FIG. 9before the distal end ring is crimped onto a balloon catheter.

FIG. 10B is a cross-section view along line 10B-10B in FIG. 10A.

FIG. 11 is a flattened, plan view of a portion of the distal end ring ofFIG. 10A after radial expansion of the distal end ring duringimplantation of the stent.

FIG. 12A is a perspective view of a distal end ring.

FIG. 12B is a cross-section view along line 12B-12B in FIG. 12A.

FIG. 13 is a flattened, plan view of a portion of the distal end ring ofFIG. 12A after radial expansion of the distal end ring duringimplantation of the stent.

FIGS. 14A and 14B are elevation views, showing a process for forming theeased corner feature according to FIGS. 9-13.

FIG. 15 is a cross-section view of a stent strut, showing a round-typeeased corner feature.

FIG. 16 is a cross-section view of a polymer stent mounted on acatheter, showing an eased corner feature on various rings of the stent.

FIG. 17 is an elevation view of an apparatus for fabricating a stent,showing a nozzle for forming an eased corner feature.

FIG. 18 is a process flow diagram showing methods for fabricating astent.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As used herein, a “stent” is a device that is placed inside the body ofa person or animal and, more particularly, within an anatomical lumen orcavity. Examples of anatomical lumen and cavities in which a stent canbe placed include without limitation arterial or venous vasculature,urethra, ureter, fallopian tubes, esophagus, and the like. Non-limitingexamples of stents within the scope of the present invention are thosewhich are self-expanding and balloon expandable, and which areconfigured for percutaneous transluminal delivery methods. Stents whichhave a finite lifetime in vivo are sometimes referred to as scaffoldsdue to their temporary nature.

As used herein, “bioresorbable” refers to a material capable beingcompletely eroded, degraded (either biodegraded and/or chemicallydegraded), and/or absorbed when exposed to bodily fluids (such as bloodor other fluid); and can be gradually resorbed, absorbed and/oreliminated by the body. Other terms such as biodegradable,bioabsorbable, and bioerodible may be found in the literature and whilethese terms have specific definitions, they are often usedinterchangeably.

As used herein, “biostable” refers to a material that is notbioresorbable.

As used herein, “abluminal surface” refers to a radially outward facingsurface.

As used herein, “luminal surface” refers to a radially inward facingsurface.

A used herein, “side surface” refers to a surface which is disposedbetween and which connects an abluminal surface and a luminal surface.

The word “distal” when used in the context of a device, refers to aportion of the device located at the front of the device or which facesin a forward direction during typical use of the device. The word“proximal” when used in the context of a device, refers to a portion ofthe device located at the rear of the device or which faces in arearward direction during typical use of the device.

Referring now in more detail to the exemplary drawings for purposes ofillustrating embodiments of the invention, wherein like referencenumerals designate corresponding or like elements among the severalviews, there is shown in FIG. 2a cross-section view of exemplary polymerstent 10 crimped onto folded balloon 30 of catheter 32 being tracked onguidewire 34. Stent struts 14 have the same height and width. Also, eachstent strut 14 has sharp corners 36, 37, 38. Stent 10 has no easedcorner feature.

As used herein, “eased corner feature” is a feature that blunts or dullsa corner on a ring to ease movement of the ring through an anatomicallumen.

FIGS. 3-5 show exemplary stent 40 having struts 14 with eased cornerfeature 42. Eased corner feature 42 enables stent 40 to move easily pastdeposits, such as calcium, on the inner surface of an anatomical lumen.Eased corner feature 42 is in the form of a bevel, also referred to as achamfer, connecting strut abluminal surface 44 and distal-facing strutside surface 46.

As used herein, “distal-facing” refers to a direction toward forward tip33 of catheter 32 on which the stent is mounted or to an orientationfacing in a forward or distal direction when the stent is being advancedthrough an anatomical lumen. The phrase “proximal-facing”refers to adirection away from forward tip 33 of catheter 32 on which the stent ismounted or to an orientation facing in a rearward direction when thestent is being pushed forward through an anatomical lumen.

Referring again to FIG. 3, stent 40 has multiple rings 20 which includeproximal end ring 20 p and distal end ring 20 d. Distal end ring 20 d isshown with eased corner feature 42 at strut abluminal surface 44 anddistal-facing strut side surface 46. There is no eased corner feature atstrut luminal surface 48 and proximal-facing strut side surface 50 ofdistal end ring 20 d.

Only distal end ring 20 d has eased corner feature 42. Eased cornerfeature 42 is absent from all other rings 20 p, 20 on stent 40. Havingeased corner feature 42 on only distal end ring 20 d minimizes anyimpact on radial strength that eased corner feature 42 may have on thestent.

In other embodiments, both distal and proximal end rings have easedcorner feature 42. For proximal end ring 20 p, eased corner feature 42would be present at strut abluminal surface 44 and proximal-facing strutside surface 50. Eased corner feature 42 would be absent from strutluminal surface 48 and distal-facing strut side surface 46 of proximalend ring 20 p. Having eased corner feature 42 on proximal end ring 20 pcan facilitate backward movement of stent 40 past deposits, such ascalcium, on the inner surface of an anatomical lumen. This can occur insituations where the stent cannot be advanced to the target lesion andthe delivery system with stent must be removed. Having eased cornerfeature 42 on only proximal and distal end rings 20 p, 20 d allows formaximization of radial strength at the stent medial segment between theproximal and distal end rings.

As shown in FIGS. 4A and 4B, eased corner feature 42 follows thethree-dimensional sinusoidal shape of distal end ring 20 d. FIG. 4Bshows a cross-section view of distal end ring 20 d taken across bendingelements 18 a, 18 b at the troughs and peaks between ring struts 16.Eased corner feature 42 is at radially outward, distal-facing corner 36of ring 20 d. Radially outward, distal-facing corner 36 is the cornerbetween distal-facing strut side surface 46 and strut abluminal surface44.

In other embodiments, eased corner feature 42 would also follow thethree-dimensional sinusoidal shape of proximal end ring 20 p in the samemanner as shown in FIGS. 4A and 4B except eased corner feature 42 wouldbe located at radially outward, proximal-facing corner 37. Radiallyoutward, proximal-facing corner 37 is the corner between proximal-facingstrut side surface 50 and strut abluminal surface 44.

In FIG. 5, eased width 52 of eased corner feature 42 is one-half strutwidth 54. As used herein, “strut width” of a particular strut is adimension from distal-facing strut side surface 46 to proximal-facingstrut side surface 50, as measured in a direction normal to any of thedistal- and proximal-facing strut side surfaces 46, 50. Eased width 52is measured along the same line as strut width 54 and is the distancefrom the extreme distal edge 47 of the eased corner feature to theextreme proximal edge 45 of the eased corner feature. Eased width 52 canbe from 1% to 100% of strut width 54. Width ratio is the ratio of easedwidth 52 to strut width 54. The width ratio can be at least 5%, at least10%, at least 25%, at least 50% or at least 75%. In some embodiments,strut width 54 is between 50 microns and 250 microns. For example, easedwidth 52 can be 96 microns while strut width 54 can be 191 microns.Other dimensions are possible.

Eased depth 56 of eased corner feature 42 is one-half strut height 58.As used herein, “strut height” of a particular strut is a dimension fromthe strut abluminal surface 44 to the strut luminal surface 48, asmeasured in a direction normal to any of the abluminal and strut luminalsurfaces 44, 48. Eased depth 56 is measured along the same line as strutheight 58 and is the distance from the extreme distal edge 47 of theeased corner feature to the extreme proximal edge 45 of the eased cornerfeature. Eased depth 56 can be from 1% to 100% of the strut height 58.Eased depth 56 can be at least 5%, at least 10%, at least 25%, at least50%, or at least 75% of strut height 58. In some embodiments, strutheight 58 is between 50 microns and 250 microns. Other dimensions arepossible.

Bevel angle 60 of eased corner feature 42 is forty-five degrees. As usedherein, “bevel angle” is the angle of incidence measured from a firstimaginary line on a flat surface of eased corner feature 42 to a secondimaginary line perpendicular to luminal surface 48. Furthermore, thefirst and second imaginary lines define an imaginary plane normal to anyof the distal- and proximal-facing strut side surfaces 46, 50. Bevelangle 60 can be any non-zero angle from 5 degrees to 85 degrees, morenarrowly from 10 degrees to 80 degrees, more narrowly from 20 degrees to70 degrees, and more narrowly from 30 degrees to 60 degrees. Otherranges for bevel angle 60 are possible.

The broken lines in FIG. 5 show two non-limiting examples in which easedcorner feature 42 is configured with different eased widths 52,different eased depths 56, and different bevel angles 60.

FIG. 6 shows apparatus 70 for making stent 40. Tubular construct 72 iscarried on cylindrical mandrel 74 which passes through the center oftubular construct 72. Mandrel 74 is removably mounted via holder 78 tomotor 80. Motor 80 is movably mounted on linear rail 82 of translationalstage 84. Tubular construct 72 can be a polymer tube from which stentstruts 14 will be cut, or tubular construct 72 can be a stent scaffoldwhich already has stent struts 14. Cutting tool 76 can be a water jet,blade, or laser. A cutting tool 76 is used to make cuts into tubularconstruct 72 to form eased corner feature 42. For example, cutting tool76 can be used to form eased corner feature 42 on either distal-facingside surface 46 or proximal-facing side surface 50 that was formedpreviously by another cutting tool. Alternatively, cutting tool 76 canalso be used to form all three distal-facing side surface 46,proximal-facing side surface 50, and eased corner feature 42.

Apparatus 70 is configured to move tubular construct 72 and cutting tool76 relative to each other to allow cuts that follow any desired stentstrut pattern. Relative motion includes a combination of rotationalmotion 86 and axial translation 88. Rotational motion 86 is accomplishedby rotation of mandrel 74 by motor 80. Axial translation 88 isaccomplished by sliding mandrel 74 together with motor 80 on rail 82. Anelectronic controller is coupled to motor 80 and translational stage 84to precisely control the relative motion, and is coupled to cutting tool76 to lower and lift a water jet or blade or to activate or deactivate alaser device at the appropriate time.

In FIG. 7, cutting tool 76 is in the form of a laser device configuredto apply beam pulses to incrementally ablate and remove polymersubstrate material from stent strut 14. The laser beam could be directedperpendicularly to abluminal surface 44 as illustrated. A bevel or othershape of eased corner feature 42 is created by varying the number ofpulses as a function of position of the laser beam relative to any ofdistal- and proximal-facing strut side surfaces 46, 50. The smallrectangles in FIG. 7 can represent the number of pulses for each axialposition of the laser beam. As the beam moves axially to the right inFIG. 7, the number of pulses increases to create a deeper cut. Forexample, with laser beam at position A, only one pulse may be used tocreate a shallow cut. With laser beam at position E, five pulses may beused to create a deeper cut. With laser beam at position J, ten pulsesmay be used to create an even deeper cut. Multiple cuts of varying depthcan create any desired shape.

In FIG. 8, cutting tool 76 is in the form of a laser device configuredto direct a beam at an oblique angle corresponding to bevel angle 60(FIG. 5). As indicated above, “bevel angle” is the angle between twolines on an imaginary plane normal to any of distal- and proximal-facingstrut side surface 46, 50. However, the orientation of strut sidesurfaces 46, 50 of any sinusoidal ring will change along thecircumference of the ring, as shown in FIG. 4A. For example,distal-facing strut side surface 46 may face distally (in the directionof arrow 62 in FIG. 4A) at peaks and troughs of bending elements 18 andface in other directions (arrows 64) at ring segments between bendingelements 18. Therefore, to maintain the same bevel angle throughout ring20 d, it will be necessary to have a degree of relative motion inaddition to rotational motion 86 and axial translation 88 (FIG. 6). Forexample, during rotational motion 86 and axial translation 88, cuttingtool 76 can be pivoted relative to tubular construct 72 so that theimaginary plane of bevel angle 60 remains perpendicular or normal todistal-facing strut side surface 46. Maintaining the same bevel anglewould help maintain the same cross-sectional area throughout the ringand, thereby, minimize variation in mechanical strength among variousparts of the ring.

In FIGS. 7 and 8, polymer substrate material is removed only fromradially outward, distal-facing corner 36 to form eased corner geometry42 on distal end ring 20 d or another ring 20, 20 p. No material isremoved from radially outward, proximal-facing corner 37 and radiallyinward corners 38.

In other embodiments, polymer substrate material is removed only fromradially outward, proximal-facing corner 37 to form eased cornergeometry 42 on proximal end ring 20 p or another ring 20, 20 d. Nomaterial is removed from radially outward, distal-facing corner 36 andradially inward corners 38

In FIGS. 9-13, eased corner feature 42 is in the form of a bevel. Easedwidth 52 (e.g., FIG. 10B) is 100% of strut width 54. Consequently, easedcorner feature 42 forms the entirety of the strut abluminal surface.Eased corner feature 42 is illustrated on distal end ring 20 d. Easedcorner feature 42 is located on radially outward, distal-facing corner36 since eased depth 56 is on distal-facing strut side surface 46.

Eased corner feature 42 of can be implemented on only distal end ring 20d as illustrated, or on both distal and proximal end rings 20 d, 20 p.Having eased corner feature 42 present on only distal end ring 20 d oronly distal and proximal end rings 20 d, 20 p provides advantagesdescribed above for FIGS. 3-4B. For embodiments in which eased cornerfeature 42 is implemented on proximal end ring 20 p, descriptions belowin reference to distal end ring 20 d would similarly apply except easedcorner feature 42 would be located on radially outward, proximal-facingcorner 37.

In FIGS. 9-10B, the strut cross-sectional area is not maintained thesame throughout end ring 20 d. The cross-sectional area is illustratedby diagonal hatching in FIG. 10B.

FIGS. 10A and 10B show how eased corner feature 42 follows thethree-dimensional sinusoidal shape of distal end ring 20 d. FIG. 10Bshows a cross-section view of distal end ring 20 d taken across bendingelements 18 a, 18 b at the troughs and peaks between ring struts 16.Bending elements 18 a, 18 b have equal strut widths 54, equal easedwidths 52, equal eased depths 56, and equal bevel angles 60. Easeddepths 56 a and 56 b are the same in terms of distance, although theratio of eased depth 56 a to strut height 58 a at proximal bendingelement 18 a is less than the ratio of eased depth 56 b to strut height58 b at distal bending element 18 b. Proximal bending element 18 a hasstrut height 58 a greater than strut height 58 b of distal bendingelement 18 b, which results in a difference in cross-sectional area.

In some embodiments, the cross-sectional area of distal end ring 20 dvaries continuously from a maximum cross-sectional area at proximalbending element 18 a to a minimum cross-sectional area at distal bendingelement 18 b. Strut height varies continuously from a maximum strutheight 58 a at proximal bending element 18 a to a minimum strut height58 b at distal bending element 18 b.

In some embodiments, the variation in cross-sectional area maycorrespond to a variation in area moment of inertia and mechanicalstrength among various segments of ring 20 d. Segments near proximalbending element 18 a can have a greater area moment of inertia andthereby be less flexible than segments near distal bending element 18 b,which may have a lower area moment of inertia. This variation can, insome embodiments, result in non-uniform expansion of ring 20 d duringimplantation, as shown in FIG. 11. With non-uniform expansion, there arelarge empty spaces 61 and small empty spaces 63 between ring struts 16.With uniform expansion, empty spaces 61, 63 would be equal.

Uniformity in the size of empty spaces 61, 63 between ring struts 16 maybe desired to improve support of surrounding tissue after implantationand provide for uniform stresses amongst bending elements 18 a, 18 b.Such uniformity may be accomplished, as shown in FIGS. 12A and 12B, byincreasing strut width 54 at distal bending element 18 b to compensatefor its smaller strut height 58 and, thereby, increase the area momentof inertia at distal bending element 18 b from that in FIG. 10B.

FIGS. 12A and 12B show how eased corner feature 42 follows thethree-dimensional sinusoidal shape of distal end ring 20 d. FIG. 12Bshows a cross-section view of distal end ring 20 d taken across bendingelements 18 a, 18 b at the troughs and peaks between ring struts 16.Bending elements 18 a, 18 b have equal bevel angles 60. Proximal bendingelement 18 a has strut height 58 a greater than strut height 58 b ofdistal bending element 18 b. To compensate for its smaller strut height,distal bending element 18 b has strut width 54 b greater than strutwidth 54 a of proximal bending element 18 a.

In some embodiments, the cross-sectional shape of ring 20 d variescontinuously from proximal bending element 18 a to distal bendingelement 18 b. The cross-sectional shape is the perimeter of the areaillustrated by diagonal hatching in FIG. 12B. Strut height 58 decreasescontinuously from a maximum strut height 58 a at proximal bendingelement 18 a to a minimum strut height 58 b at distal bending element 18b. Strut width 54 increases continuously from a minimum strut width 54 aat proximal bending element 18 a to a maximum strut width 54 b at distalbending element 18 b.

The variation in strut height 58 and strut width 54 described above, incombination with eased corner feature 42, can allow for increaseduniformity in the size of empty spaces between ring struts 16 afterexpansion of stent 40 during implantation, as shown in FIG. 13. In someembodiments, strut height 58 decreases continuously from a maximum strutheight 58 a at proximal bending element 18 a to a minimum strut height58 b at distal bending element 18 b, and strut width 54 increasescontinuously from a minimum strut width 54 a at proximal bending element18 a to a maximum strut width 54 b at distal bending element 18 b, suchthat angles 66 between one pair of ring struts 16 is the same as thatfor another pair of ring struts 16 on ring 20 d. The size of emptyspaces 61, 63 may also be uniform.

The bevel-type eased corner feature 42 of FIGS. 9-13 can be fabricatedby thinning a portion of tubular construct 72 and then forming any ofproximal and distal end rings 20 p, 20 d on thinned portion 90.

As used herein, “bevel-type eased corner feature” includes one or moresurfaces each defined by a bevel angle. The surfaces can be smooth orhave some irregularities, such as the step-wise profile shown in thehighly enlarged illustration in FIG. 7. For example, bevel-type easedcorner feature can include a first surface having a bevel angle of 60degrees and a second surface having a bevel angle of 30 degrees. Strutabluminal surface 44 would connect to the first surface, which wouldconnect to the second surface, which would connect to distal-facingstrut side surface 46.

As shown in FIG. 14A, abluminal surface 44 of tubular construct 72 ismachined with thinning tool 92. Tubular construct 72 is mounted onmandrel 74 of the apparatus shown in FIG. 6 to enable relative movementbetween thinning tool 92 and tubular construct 72. Thinning tool 92modifies abluminal surface 44 to form thinned portion 90. Thinning tool92 can be a blade, laser device for ablating material as described forFIG. 7 or FIG. 8, or an abrasive surface that scrapes, sands, orpolishes material away. Thinning tool 92 may include a combination of ablade, laser device, and/or abrasive surface to create a smooth finish.Within thinned portion 90, abluminal surface 44 tapers radially inwardso that the radial wall thickness of the tubular construct 72 graduallydecreases to a point of minimum wall thickness. Thinned portions 90 maybe symmetrical as shown in FIG. 14B or they may be non-symmetrical.

As shown in FIG. 14B, tubular construct 72 with thinned portion 90 isprocessed using cutting tool 76. Cutting tool 76 cuts material away fromtubular construct 72 to form a stent scaffold having a plurality ofrings 20 connected by link struts 22. A circumferential strip region 96where an end ring will be cut is registered or aligned within thinnedportion 90 so that the end ring will have eased corner feature 42according to any of FIGS. 9-13.

In FIG. 15, eased corner feature 42 is in the form of a rounded corneror fillet. This round-type eased corner feature 42 can be substitutedfor the bevel-type eased corner feature of FIGS. 3-4B to form additionalembodiments of the present invention.

As used herein, “round-type eased corner feature” includes one or morecurved surfaces, each defined by a convex shape that meets any of strutabluminal surface 44, distal-facing strut side surface 46, orproximal-facing strut side surface 50. The curved surfaces can be smoothor have some irregularities, such as may arise due to incremental laserablation as described in FIG. 7. For example, strut abluminal surface 44can meet a first convex surface, and distal-facing strut side surface 46can meet a second convex surface. Optionally, the first and secondconvex surfaces can meet. Optionally, an intervening surface is disposedbetween and meets the first and second convex surfaces.

It will be appreciated that eased corner feature 42 is not limited tothe bevel-type and round-type shapes illustrated herein. For example,eased corner feature 42 can be a combination of two or more flatsurfaces with a curved surface between two adjacent flat surfaces.

In FIG. 15, round-type eased corner feature 42 is a quarter round. Asused herein, a quarter round has a cross-sectional edge that is aquarter circle with opposite ends of the quarter circle being tangentwith strut abluminal surface 44 and distal-facing strut side surface 46(or with proximal-facing strut side surface 50 if implemented onproximal end ring 20 p, for example).

Round-type eased corner feature 42 can be defined by its radius. In FIG.15, the solid-line showing of round-type eased corner feature 42 hasradius 98 that is 25% of strut width 54. Radius 98 can be at least 5%,at least 10%, at least 25%, at least 50%, at least 75%, or 100% of strutwidth 54. Eased width 52 and depth 56 of eased corner feature 42 can beas described above for FIG. 5.

The broken lines in FIG. 15 show two non-limiting examples in whicheased corner feature 42 is configured with different eased widths 52,different eased depths 56, and different radii 98.

As shown in FIG. 16, round-type eased corner feature 42 can be presenton distal-facing strut side surface 46 on all rings 20, 20 p, 20 d ofstent 40.

As shown in FIG. 17, round-type eased corner feature 42 can be formed bynozzle 100 oriented in a rearward facing direction. For example, a stentscaffold having a plurality of rings 20 connected by link struts 22 canbe mounted on mandrel 74 of the apparatus described in FIG. 6. Nozzle100 is configured to eject abrasive media 102 that removes polymersubstrate material from radially outward, distal-facing corners 36 (FIG.15) to form round-type eased corner feature 42 shown in FIGS. 15 and 16.Mandrel 74 protects strut luminal surfaces 44 so that no polymersubstrate material is remove from radially inward corners 38. Round-typeeased corner feature 42 can be limited to distal end ring 20 d, proximalend ring 20 p, or other selected rings 20 by directing the abrasivemedia only at these rings or by placing a mask or shield over the ringswhich are not to be exposed to abrasive media 102.

In other embodiments, nozzle 100 is oriented in a forward facingdirection to eject abrasive media 102 that removes polymer substratematerial from outward, proximal-facing corners 37 to form round-typeeased corner feature 42 on proximal-facing strut side surfaces 50 of anyone or more of rings 20, 20 p, 20 d.

Abrasive media include without limitation solid particles of silica,glass, alumina, garnet, sodium bicarbonate, and silicon carbide. Thesize of the particles can be selected based on strut width and desiredwidth of eased corner feature. An average particle size can be from 1micron to 25 microns. Embedding of particles of abrasive media intostruts can be avoided or minimized through optimization by adjusting anyone or more of the size of the particles, velocity of the particles, andmaterial density of the particles. Particles having a material densitythat is closer to or less than that of the polymer substrate material ofstent 40 are expected to be less likely to become embedded thanparticles having a greater material density. For example, a materialdensity less than 1.25 gm/cm³ may be appropriate for use when thepolymer substrate material of the stent comprises poly(L-lactide)(“PLLA”).

Abrasive media 102 can be carried by a stream of dry air, nitrogen, orother inert gas, such as argon. After ejection of the abrasive media iscompleted, the stent can be rinsed with liquid to remove abrasive mediaon the stent. The liquid can be a solvent of the abrasive media tofacilitate removal of any embedded particles of the abrasive media. Forexample, particles of sodium bicarbonate can be used as the abrasivemedia, and water can be used to rinse and dissolve away any particles ofsodium bicarbonate that might have become embedded in the stent struts.

Alternatively, particles of abrasive media 102 can be carried by astream of liquid ejected from nozzle 100. The liquid may comprise anaqueous solution or a solvent of the particles.

Round-type eased corner feature 42 can also be formed by directing aspray of liquid which is a partial solvent of the polymer substratematerial of stent struts 14. The impact of the spray will deform andsoften the polymer substrate material and dissolve away a small amountof the polymer substrate material from radially outward, distal-facingcorners to form round-type eased corner feature 42 shown in FIGS. 15 and16.

Round-type eased corner feature 42 can also be formed by buffingradially outward, distal-facing corners 36 and/or radially outward,proximal-facing corners 37. Buffing can be performed using buffing wheel104 which rotates about axis 106 and which is coated with abrasive mediaor fibers. Buffing wheel 104 can be oriented at an oblique angle,similar to the orientation of nozzle 100.

As shown in FIGS. 3-5, 9-10B, 12A, 12B, 16, 15, eased corner feature isabsent from all radially inward corners 38 of all rings 20 p, 20, 20 dof stent 40. For all rings 20 p, 20, 20 d, strut luminal surface 48 andproximal-facing strut side surface 50 meet to form sharp radially inwardcorners 38 having an interior angle 39 (FIGS. 5 and 15) of ninetydegrees. In other embodiments, sharp radially inward corners 38 haveinterior angles 39 of less than 100 degrees.

In further embodiments, any of the eased corner feature 42 describedabove can be implemented on stent 40, such that: (1) eased cornerfeature 42 is located on radially outward, distal-facing corner 36 butnot on radially outward, proximal-facing corner 37 and not on radiallyinward corners 38; (2) eased corner feature 42 is located on radiallyoutward, proximal-facing corner 37 but not on radially outward,distal-facing corner 36 and not on radially inward corners 38; or (3)eased corner feature 42 is located on radially outward, distal-facingcorner 36 and radially outward, proximal-facing corner 37 but not onradially inward corners 38. Case (1), (2), or (3) can be applied to allrings 20 p, 20, 20 d of stent 40. Case (1), (2), or (3) above can beapplied to proximal end ring 20 p and distal end ring 20 d of stent 40,while all other rings 20 of stent 40 have no eased corner geometry. Case(1), (2), or (3) above can be applied to proximal end ring 20 p, whileall other rings 20 and 20 d of stent 40 have no eased corner geometry.Case (1), (2), or (3) above can be applied to distal end ring 20 d,while all other rings 20 p and 20 of stent 40 have no eased cornergeometry.

In any one or more of the embodiments above, tubular construct 72 (fromwhich stent 40 is made) is a hollow polymer cylinder that has beenradially expanded as described in U.S. Pat. No. 8,002,817. The hollowpolymer cylinder can be made by extruding one or more bioresorbablepolymers through a circular die to form a precursor tube made entirelyof the one or more bioresorbable polymers. The precursor tube is thenheated and radially expanded, such as by introduction of pressurized airinto the tube, in order alter the orientation of molecular polymerchains and thereby increase fracture toughness and strength. Somematerial is then cut away from the radially expanded tube, such as bylaser machining using the apparatus according to any of FIGS. 6, 14A and14B. The remaining material is in the form of a tubular scaffoldcomprising various stent struts 14. Eased corner feature 42 can beformed during the laser machining process that forms stent struts 14.Alternatively, eased corner feature 42 can be formed during a secondaryprocess performed after a primary laser machining process that formsstent struts 14.

A method for fabricating a stent is shown in FIG. 18 with reference todevices described above although the method is not limited to suchdevices. It will be appreciated that the method can be performed usingother types of devices.

In FIG. 18, a method of fabricating a stent includes providing tubularconstruct 72 in block 110. The tubular construct is made of a polymersubstrate material. Block 110 optionally includes extruding a polymertube in block 112, and optionally followed by heating and radiallyexpanding the extruded polymer tube in block 114. Block 114 may alsoencompass heating or annealing the polymer tube with no radial expansionof the polymer tube. Alternative methods of providing the tubularconstruct include dip forming, injection molding polymer material, androlling a flat sheet of polymer material.

After block 110, the method includes cutting the tubular construct inblock 116 to form interconnected stent struts 14 with eased cornerfeature 42. One or more tools can be used to form stent struts 14 withone or two eased corner features 42. For example, a single cutting tool76 in any of FIGS. 6-8 can be used to form sharp radially inward corners38 on struts 14 and to form one or two eased corner features 42 atradially outward corners 36, 37. When forming eased corner feature 42 atone radially outward corner 36 (or 37), sharpness of the other radiallyoutward corner 37 (or 36) and radially inward corners 38 is maintained.

As an alternative to block 116, the method includes cutting tubularconstruct 72 in block 118 to form stent struts 14. A first tool can beused to form stent struts 14. Stent struts 14 would have sharp radiallyoutward corners 36, 37 and sharp radially inward corners 38. The firsttool can be, for example, cutting tool 76 in FIG. 6. After bock 118, themethod includes modifying the stent struts in block 120 to form easedcorner feature 42 to remove sharpness at any of radially outward corners36, 37. Modification of stent struts 14 includes removing or softeningpolymer substrate material at any of radially outward corners 36, 37.When modify stent struts 14 to have eased corner feature 42 at oneradially outward corner 36 (or 37), sharpness is maintained at the otherradially outward corner 37 (or 36) and at radially inward corners 38. Asecond tool can be used to modify stent struts 14. The second tool canbe any one or a combination of cutting tool 76 in FIGS. 7 and 8, nozzle100 in FIG. 17, and buffing wheel 104 in FIG. 17.

As an alternative to blocks 116-120, the method includes modifyingtubular construct 72 in block 122 to have a non-uniform wall thickness.For example, thinning tool 92, such as shown in FIG. 14A, can be used toremove material from abluminal surface 44 of tubular construct 72.Thinning tool 92 removes polymer substrate material without penetratingentirely through tubular construct 72. After block 122, the methodincludes cutting the tubular construct in block 124 to forminterconnected stent struts 14 with eased corner feature 42. Easedcorner feature 42 is present as soon as cutting tool penetrates entirelythrough tubular construct 72 to form proximal-facing side surfaces 50and distal-facing side surfaces 46. That is, eased corner feature 42 isformed using a combination of the thinning tool followed by the cuttingtool. For example, cutting tool 76, such as shown in FIG. 14B, can beused. When cutting side surfaces 46, 50 into thinned portion 90 oftubular construct 72, cutting tool 76 forms sharp radially inwardcorners 38 and forms eased corner feature 42. Cutting thinned portion 90results in formation of eased corner feature 42 while sharpness ismaintained at radially inward corners 38 and at one of the radiallyoutward corners 37, 36.

In the embodiments above, stent 40 is made of a polymer substratematerial. The polymer substrate material can be a biostable polymersubstrate material or a bioresorbable polymer substrate material. In anyone or more embodiments above, the bioabsorbable polymer material is amaterial selected from the group consisting of poly(L-lactide) (“PLLA”),poly(L-lactide-co-glycolide) (“PLGA”), poly(D,L-lactide-co-glycolide),poly(L-lactide-co-caprolactone), poly(glycolide-co-caprolactone) andpoly(L-lactide-co-D-lactide) (“PLLA-co-PDLA”). Examples of polymers forthe polymer substrate material include without limitation the polymersdescribed in U.S. Pat. No. 8,002,817.

A surface coating containing a therapeutic agent, a polymer, solvent, ora combination thereof, can be applied on the polymer substrate material.The surface coating is not a substrate material of stent 40. Therapeuticagents include without limitation drugs and substances that, whenadministered in therapeutically effective amounts, have a therapeuticbeneficial effect on the health and well-being of the patient orsubject. Therapeutic agents include without limitation ananti-proliferative, anti-inflammatory or immune modulating,anti-migratory, anti-thrombotic or other pro-healing agent or acombination of two or more thereof. Therapeutic agents include withoutlimitation those described in U.S. Publication Nos. 2010/0244305.Polymers include without limitation those described in U.S. Pat. No.8,002,817.

Eased corner feature 42 can be implemented on polymer stent rings ofvarious shapes, such as rings disclosed in U.S. Pat. Nos. 8,002,817,8,303,644, 8,388,673, and 8,323,760, and in U.S. Publication Nos.2010/0244305 and 2013/0085563. For example, stent rings can have asinusoidal shape, as shown in FIGS. 4A, 10A and 12A. The sinusoidalshape is formed by an axially undulating arrangement of ring strutsjoined together by bending elements. The sinusoidal shape includes aseries of repeating S-curves. In FIGS. 4A, 10A, and 12A, an S-curve isformed by first link strut 16 at the top of the figure, which isconnected by distal bending element 18 b to second link strut 16, whichis connected by proximal bending element 18 a to third link strut 16.Stent rings need not be sinusoidal or undulating. For example, a stentring can have the shape of a cylindrical band such as ring member 40 inFIG. 3 of U.S. Publication No. 2010/0244305.

While several particular forms of the invention have been illustratedand described, it will also be apparent that various modifications canbe made without departing from the scope of the invention. It is alsocontemplated that various combinations or subcombinations of thespecific features and aspects of the disclosed embodiments can becombined with or substituted for one another in order to form varyingmodes of the invention. Accordingly, it is not intended that theinvention be limited, except as by the appended claims.

What is claimed is:
 1. An implantable stent comprising: a first ring anda second ring, each of the first ring and the second ring made of apolymer substrate material forming a strut abluminal surface, a strutluminal surface, a distal-facing strut side surface, and aproximal-facing strut side surface, wherein the distal-facing strut sidesurfaces are normal to and meet strut luminal surfaces to form radiallyinward corners not dulled or blunted on the first ring and the secondring, a corner feature is formed in the polymer substrate material ofthe first ring, and the corner feature is located at a radially outward,distal-facing corner of the first ring and, wherein the corner featureblunts or dulls the radially outward, distal facing corner and isdefined by a surface between the strut abluminal surface and thedistal-facing strut side surface, and wherein the surface defining thecorner feature has a step-wise profile.
 2. The implantable stent ofclaim 1, wherein the corner feature has a corner feature width and acorner feature depth, the corner feature width is at least 5% of strutwidth, and the corner feature depth is at least 5% of strut height. 3.The implantable stent of claim 1, wherein the first ring is a distal endring.
 4. The implantable stent of claim 1, wherein a corner feature isformed in the polymer substrate material of the second ring and definedby a surface having a step-wise profile.
 5. The implantable stent ofclaim 4, wherein the corner feature of the second ring is located on aradially-outward, distal-facing corner of the second ring.
 6. Theimplantable stent of claim 4, wherein the corner feature of the secondring is located on a radially-outward, proximal-facing corner of thesecond ring.
 7. The implantable stent of claim 6, wherein the first ringis a distal end ring, and second ring is a proximal end ring.
 8. Theimplantable stent of claim 1, further comprising a plurality ofadditional rings each of which made of the polymer substrate material,wherein a corner feature is formed in the polymer substrate material atradially outward, distal-facing corner of each of the additional rings,and the corner feature is defined by a surface having a step-wiseprofile.
 9. The implantable stent of claim 1, wherein the first ringincludes ring struts joined together by a proximal bending element and adistal bending element, wherein the ring struts, the proximal bendingelement, and the distal bending element form a sinusoidal shape, whereina width ratio is a ratio of corner feature width of the corner featureto strut width, and the width ratio of the first ring is the same at theproximal bending element and at the distal bending element, and a cornerfeature depth of the corner feature of the first ring is the same at theproximal bending element and at the distal bending element.
 10. Theimplantable stent of claim 9, wherein the corner feature width at theproximal bending element and at the distal bending element is less than100% of strut width.
 11. The implantable stent of claim 9, wherein thecorner feature width at the proximal bending element and at the distalbending element is 100% of strut width.
 12. The implantable stent ofclaim 1, wherein the step-wise profile includes a plurality of steps,and each step defined by having an elevation different from that of anadjacent one of the steps.
 13. The implantable stent of claim 12,wherein each step is formed by a laser pulse directed perpendicular tothe strut abluminal surface.
 14. The implantable stent of claim 1,wherein the radially inward corners have no beveled or rounded edge.